In exploration and production activities in the oil and gas industry, cable-operated so-called jars are used when downhole equipment that is stuck is to be retrieved from a borehole. The jar, which, during the freeing operation, is connected to the downhole equipment that is to be freed, includes a jarring mechanism which is releasable by the cable, which connects the jar to a surface installation, being subjected to a prescribed tensioning, and the jarring mechanism strikes against a portion of the jar, thereby applying a stroke impulse to the downhole equipment. According to the prior art, the impact force of the jar is set while the jar is on the surface, and if the set impact force is insufficient to free the downhole equipment from the surrounding structures even after repeated strokes, the jar will have to be released from the downhole equipment, retrieved to the surface, readjusted, lowered down the borehole again, reconnected to the downhole equipment in order then to repeat the jarring operation. It also happens that the set impact force is too large in relation to what is needed, and in such a situation, the downhole equipment may be subjected to unduly heavy strains.
The jarring mechanism in a jar may, in principle, include a dividable mandrel which is accommodated in a housing. In an operative position, the housing is fixed to the downhole equipment by means of a coupling arranged at an end portion of the housing (adapted for a so-called fishing neck, for example). The cable is attached to an end portion of the mandrel remote from the coupling of the housing. By pulling on the cable, the mandrel is moved into engagement between a lock and a portion of the housing while, at the same time, a spring arrangement is tightened by abutment against a first shoulder in the housing. A mandrel coupling, which may form part of the lock, forms a releasable coupling between an upper mandrel section and a lower mandrel section. An adjusting mechanism, for example a nut, functions as a displaceable lower abutment for the spring arrangement.
When the cable is stretched further, the coupling is released, and the upper mandrel section which is provided with a projecting hammer portion, strikes against a second shoulder (anvil) in the housing. The reaction force is transmitted to the downhole equipment which, if the force is large enough, is knocked loose from the surrounding structures.
In addition to the adjusting mechanism of the jar being able to change the stroke characteristics of the jar, the spring arrangement can also be replaced to give the jar a completely different stroke characteristics, for example outside the adjustment range offered by the adjusting mechanism.
U.S. Pat. No. 3,735,827 discloses a hydraulically operated jar, in which a hydraulic fluid in a compression chamber bypasses a compression piston via an unrestricted flow passage us which is normally dosed by a valve. The valve is opened by means of a trigger mechanism which is adjustable in order to vary the tension that must be applied to a cable to open the valve.
U.S. Pat. No. 6,182,775 discloses a jar which includes an elongated housing with a fluid bore extending between the end portions of the housing. Two pistons are displaceably arranged in the housing, each provided with a seat. A valve ball which can be pumped down to the jar through a coiled tubing, for example, may rest against the seat of the upper piston. A dart is arranged between the pistons. A trip mechanism separates the dart from the lower piston at a certain overpressure. Once the plug is separated from the lower piston, the lower piston is moved towards an abutment in the delivering an upward blow to the housing.